Rapid prototyping of models includes the making of three dimensional solid objects in accordance with a specified design, with the design usually comprising mathematical data from a three dimensional solid computer-aided design system. Rapid prototyping systems create solid objects by:
sequential photopolymerization of layers of a monomer, PA1 milling away material, PA1 laser fusing of particulate, PA1 sequential extrusion of a thermoplastic, PA1 laminating scribed layers of paper, PA1 jetting thermally solidifiable wax or metal, PA1 laser enhanced chemical vapor deposition, PA1 brazing together pre-machined plates, PA1 by jetting binder onto ceramic powders, PA1 other techniques. PA1 a) computing a sequence of commands required to produce the predetermined shape of the three-dimensional physical object; PA1 b) dispensing a thermally solidifiable material in a fluid state from an extruder into a build environment as prescribed by the sequence of commands; PA1 c) maintaining during step b) the build environment, at least in a vicinity of the extruder, within a predetermined temperature range, the temperature range being above a solidification temperature of the thermally solidifiable material; PA1 d) simultaneously with the dispensing step b) and in response to the sequence of commands, mechanically generating relative movement between the extruder and the build environment, so that the material accumulates to form the three-dimensional physical object; PA1 e) concurrently with step d), adjusting temperatures within the build environment differentially so that the solidifiable material, upon which additional solidifiable material has accumulated, is cooled below a solidification temperature thereof; and PA1 f) further solidifying the object by cooling the object below the solidification temperature.
A preferred rapid prototyping system creates solid models by depositing thermally solidifiable materials. In these processes, a flowable material is sequentially deposited on a seed, a substrate, or on previously deposited thermoplastic material. The material solidifies after it is deposited and is thus able to incrementally create a desired form. Examples of thermally solidifiable systems include fused deposition modeling, wax jetting, metal jetting, consumable rod arc welding, and plasma spraying.
Since most deposition materials change density with temperature, especially as they transition from a fluid to a solid, thermally solidifiable material rapid prototyping systems share the challenge of minimizing geometric distortions of the product prototypes that are produced by these density changes. Thermally solidifiable systems are subject to both "curl" and "plastic deformation" distortion mechanisms. Curl is manifest by a curvilinear geometric distortion which is induced into a prototype during a cooling period. The single largest contributor to such a geometric distortion (with respect to prototypes made by the current generation of rapid prototyping systems which utilize a thermally solidifiable material) is a change in density of the material as it transitions from a relatively hot flowable state to a relatively cold solid state.
For the simple case where an expansion coefficient is independent of temperature, the nature and magnitude of geometric distortion of sequentially applied planar layers can be estimated. Assume a linear thermal gradient dT/dz is present in a material when it is formed into a plate of thickness h in the z direction, and that the material has a constant thermal expansion coefficient .alpha.. The z direction is generally orthogonal to a support surface on which the plate is constructed. If the plate is subsequently allowed to come to some uniform temperature, it will distort, without applied stress, to form a cylindrical shell of radius r where: EQU r=(.alpha.*dT/dz).sup.-1 ( 1)
Curl C is defined as the inverse of the radius of curvature: C=1/r. An example of positive curl is shown in FIG. 1. Sequential layers of a thermoplastic material 104 are deposited on a base 102, using a moving extruder 106. As is typical in thermally solidified rapid prototypes, a series of layers are deposited sequentially in the z direction (i.e., the direction orthogonal to base 102), with the last layer deposited always having the highest temperature. Such an additive process typically results in a geometrically accurate part which contains a thermal gradient. As the part subsequently cools and becomes isothermal, the part distorts as a result of a curling of the ends of long features.
If it is desired to make prototypes with a maximum horizontal length L and a maximum allowable geometric distortion .delta., the maximum allowable temperature gradient within the part, as it is being formed, is as follows : EQU (dT/dz).sub.max =8.delta./L.sup.2 .alpha. (2)
For example, to make 12 inch long parts to a tolerance of 0.030 inches, with a thermoplastic having an expansion coefficient of 90.times.10.sup.-6 per degree Centigrade, the maximum allowable thermal gradient in the part during formation is 18.degree. C. per inch. Unfortunately, thermal gradients are usually much greater than 18.degree. C. per inch in the vicinity of a part where fluid material is solidifying.
Techniques exist to reduce the impact of curl. One technique involves the heating of the ambient build environment to reduce the possible temperature differences. Another technique is to carefully choose build materials which exhibit lowest possible thermal expansion coefficients. Yet another technique is to deposit the build material at the lowest possible temperature.
Plastic deformation is a second phenomenon, unrelated to the thermal expansion coefficient, of a build material which can also produce distortion in a thermally solidifying prototype. Consider the fused deposition modeling apparatus shown in FIGS. 2A and 2B. In both cases, a flowable material flows out of a heated nozzle 106 and is solidifying on previously deposited, solidified material 104. In FIG. 2A, nozzle 106 is moving upwardly as it deposits material 104, while in FIG. 2B nozzle 106 moves downwardly. In FIG. 2A, the material emerging from nozzle 106 sees less than a 90.degree. bend as it is deposited, while in FIG. 2B it sees more than a 90.degree. bend. Experimentally, more distortion arises from the configuration of FIG. 2B than from FIG. 2A. Further, the effect is more pronounced for lower deposition temperatures. This is attributed to inelastic deformation of the elastic component of the material. The distortion is similar to the curl created in a piece of paper by dragging the paper over a sharp right angle bend.
The art is replete with various solid modeling teachings. For instance, U.S. Pat. No. 5,121,329 to Crump, and assigned to the same Assignee as this Application, describes a fused deposition modeling system. While the Crump system incorporates a heated build environment, it requires that the deposited material be below its solidification temperature, as subsequent layers of material are added. U.S. Pat. No. 4,749,347 to Vilavaara and U.S. Pat. No. 5,141,680 to Almquist et al. describe rapid prototyping systems that incorporate flowable, thermally solidifying material. Both patents teach a build environment that is maintained at and below the solidification temperature of the extrusion material.
Accordingly, it is an object of the invention to provide an improved method for rapid prototyping, wherein the method employs a thermally solidifiable material.
It is another object of the invention to provide an improved method for rapid prototyping which improves the geometric accuracy and fidelity of resulting parts.
It is a further object of the invention to provide an improved method for rapid prototyping which uses thermally solidifiable material which achieves reductions in internal stresses created in prototypes.